Gated 3d camera

ABSTRACT

A camera for determining distances to a scene, the camera comprising: a light source comprising a VCSEL controllable to illuminate the scene with a train of pulses of light having a characteristic spectrum; a photosurface; optics for imaging light reflected from the light pulses by the scene on the photosurface; and a shutter operable to gate the photosurface selectively on and off for light in the spectrum.

FIELD

The technology relates to gated 3D cameras, and methods and apparatusfor acquiring 3D images of a scene using a gated 3D camera.

BACKGROUND

Three-dimensional (3D) optical imaging systems, hereinafter referred toas “3D cameras”, that are capable of providing distance measurements toobjects and points on objects that they image, are used for manydifferent applications. Among these applications are profile inspectionsof manufactured goods, CAD verification, robot vision, geographicsurveying, and imaging objects selectively as a function of distance.

Some 3D cameras provide simultaneous measurements to substantially allpoints of objects in a scene they image. Generally, these 3D camerascomprise a light source, typically comprising an array of edge emittinglaser diodes, which is controlled to provide pulses of light forilluminating a scene being imaged, and a gated imaging system forimaging light from the light pulses that is reflected from objects inthe scene. The gated imaging system comprises a camera having aphotosensitive surface, hereinafter referred to as a “photosurface”,such as a CCD or CMOS photosurface and a gating means for gating thecamera open and closed, such as an optical shutter or a gated imageintensifier. The reflected light is registered on pixels of thephotosurface of the camera only if it reaches the camera when the camerais gated open.

To image a scene and determine distances from the camera to objects inthe scene, the light source is generally controlled to radiate a trainof light pulses to illuminate the scene. For each radiated light pulsein the train, following an accurately determined delay from the timethat the light pulse is radiated, the camera is gated open for a periodhereinafter referred to as a “gate”. Light from the light pulse that isreflected from an object in the scene is imaged on the photosurface ofthe camera if it reaches the camera during the gate. Since the timeelapsed between radiating a light pulse and the gate that follows it isknown, the time it took imaged light to travel from the light source tothe reflecting object in the scene and back to the camera is known. Thetime elapsed is used to determine the distance to the object.

In some “gated” 3D cameras, only the timing between light pulses andgates is used to determine distance from the 3D camera to a point in thescene imaged on a pixel of the photosurface of the camera. In others, anamount of light registered by the pixel during the time that the camerais gated open is also used to determine the distance. The accuracy ofmeasurements made with these 3D cameras is a function of the rise andfall times of the light pulses and their flatness, and how fast thecameras can be gated open and closed.

Gated 3D cameras and examples of their uses are found in European PatentEP1214609 and in U.S. Pat. No. 6,057,909, U.S. Pat. No. 6,091,905, U.S.Pat. No. 6,100,517, U.S. Pat. No. 6,327,073, U.S. Pat. No. 6,331,911,U.S. Pat. No. 6,445,884, and U.S. Pat. No. 6,794,628, the disclosures ofwhich are incorporated herein by reference. A 3D camera using a pulsedsource of illumination and a gated imaging system is described in“Design and Development of a Multi-detecting two Dimensional RangingSensor”, Measurement Science and Technology 6 (September 1995), pages1301-1308, by S. Christie, et al., and in “Range-gated Imaging for NearField Target Identification”, Yates et al, SPIE Vol. 2869, p 374-385which are herein incorporated by reference. Another 3D camera isdescribed in U.S. Pat. No. 5,081,530 to Medina, which is incorporatedherein by reference. A 3D camera described in this patent registersenergy in a pulse of light reflected from a target that reaches thecamera's imaging system during each gate of a pair of gates. Distance toa target is determined from the ratio of the difference between theamounts of energy registered during each of the two gates to the sum ofthe amounts of energy registered during each of the two gates.

R&D efforts to enhance accuracy of measurements provided by 3D cameras,are typically invested in developing methods and devices for reducingrise times, fall times, and widths of light pulses transmitted toilluminate a scene and corresponding gates during which light reflectedfrom the pulses by the scene is imaged.

SUMMARY

An aspect of some embodiments of the technology relates to providing animproved gated 3D camera.

An aspect of some embodiments of the technology, relates to providing agated 3D camera having improved spatial accuracy with which features ina scene imaged by the camera are located.

An aspect of some embodiments of the technology relates to providing agated 3D camera having an improved light source for illuminating scenesimaged by the camera.

An aspect of some embodiments of the technology, relates to providing agated 3D camera having improved matching of a light source used toilluminate a scene imaged by the camera with a camera shutter that gatesthe camera photosurface on which light from the light source reflectedby the scene is imaged.

According to an aspect of some embodiments of the technology, the lightsource comprises a VCSEL having a structure that is modified relative totypical VCSEL structures and is characterized by a relatively wide lasercavity.

The inventors have determined that accuracy of distances provided by agated 3D camera is dependent on a convolution of a spectrum of thecamera light source and a contrast ratio (CR) function of the camerashutter. The contrast ratio CR defines dependency of contrast ratio ofthe camera shutter on optical wavelength. For a given wavelength, CR isa ratio between a relatively high transparency of the shutter at thewavelength when the shutter is open, to a relatively low transparency ofthe shutter for light at the wavelength when the shutter is closed.Wavelengths for which the shutter can practically be used to shutterlight are wavelengths for which its CR function is greater than one and,generally, substantially greater than one. A band of wavelengths for ashutter for which CR is greater than one is referred to as an “operatingband” of the shutter.

For given rise times, fall times and widths of light pulses and gates,accuracy of distance measurements provided by a 3D gated camera canadvantageously be improved by matching the shutter CR function and lightsource spectrum to maximize the convolution of the CR function andspectrum. Generally, matching a light source spectrum to a shuttersubstantially centers the spectrum in the operating band of the shutter.For convenience of presentation, the convolution between the shutter CRand the light source spectrum is referred to as a contrast intensity(CI). A normalized contrast intensity (CIN), the CI normalized to atotal optical energy in a pulse of light provided by the light source,is conveniently used as a measure of a match between the light sourceand the shutter.

In general, periodic short-term changes in temperature of a gated 3Dcamera light source are generated relative to an ambient operatingtemperature of the camera during periods in which the light source isactivated to illuminate a scene imaged by the camera. For conventionalgated 3D camera light sources that typically comprise an edge emittinglaser diode, the temperature changes cause wavelength shifts in thelight source spectrum relative to the CR function of the camera shutter.Width of the spectrum relative to the CR operating band of the shutteris generally such that the wavelength shifts substantially misalign thespectrum and the CR function, reduce normalized contrast intensity CINof the camera and increase thereby a bias error in distancemeasurements.

Whereas conventional light sources, such as vertical cavity surfaceemitting light emitting lasers (VCSELs), that are characterized byrelatively narrow spectra are known, such light sources are typicallyused for low power applications such as communication systems. Theygenerally do not produce sufficient amounts of light to make themadvantageous for use in 3D gated cameras.

The inventors however have determined, that a VCSEL can be modified toincrease its optical output by broadening its laser cavity. Whereas,broadening the VCSEL laser cavity causes width of the VCSEL spectrum toincrease, the spectrum is still, generally, substantially narrower thanthat typically provided by conventional edge emitting laser diodes.Furthermore, wavelength shifts in the output spectrum of a VCSEL perdegree change in temperature are substantially less than that of aconventional edge emitting laser diode. As a result, a light sourcecomprising a modified VCSEL in accordance with an embodiment of thetechnology, for use in a gated 3D camera, provides relatively improvedmatching of the light source and the camera shutter. The modified VCSELlight source results in a CIN for the camera that is relatively largeand relatively insensitive to temperature change of the light source. Inan embodiment of the technology, the light source comprises an array ofmodified VCSELs.

There is therefore provided in accordance with an embodiment of thetechnology, a camera for determining distances to a scene, the cameracomprising: a light source comprising a VCSEL controllable to illuminatethe scene with a train of pulses of light having a characteristicspectrum; a photosurface; optics for imaging light reflected from thelight pulses by the scene on the photosurface; and a shutter operable togate the photosurface selectively on and off for light in the spectrum.Optionally, the characteristic spectrum has a FWHM width equal to orgreater than about 1.5 nm. Optionally, the characteristic spectrum has aFWHM width equal to or greater than about 2.0 nm. Optionally, thecharacteristic spectrum has a FWHM width equal to or greater than about2.5 nm.

In some embodiments of the technology, the VCSEL has a lasing cavitycharacterized by a diameter about equal to or greater than 20 microns.In some embodiments of the technology, the VCSEL has a lasing cavitycharacterized by a diameter about equal to or greater than 25 microns.In some embodiments of the technology, a normalized convolution of theshutter CR and characteristic spectrum is greater than or equal to about10 for a temperature difference between the shutter and light sourceless than or equal to about 20° C. In some embodiments of thetechnology, a normalized convolution of the shutter CR andcharacteristic spectrum is greater than or equal to about 12 for atemperature difference between the shutter and light source less than orequal to about 20° C.

In some embodiments of the technology, the light source operates at apower level about equal to or greater than 12 Watts to illuminate thescene with the train of light pulses. Optionally, the power level isabout equal to or greater than 15 Watts. Optionally, the power level isabout equal to or greater than 18 Watts.

BRIEF DESCRIPTION OF FIGURES

Non-limiting examples of embodiments of the technology are describedbelow with reference to figures attached hereto that are listedfollowing this paragraph. Identical structures, elements or parts thatappear in more than one figure are generally labeled with a same numeralin all the figures in which they appear. Dimensions of components andfeatures shown in the figures are chosen for convenience and clarity ofpresentation and are not necessarily shown to scale.

FIG. 1 schematically shows a 3D gated camera operating to determinedistance to a scene in accordance with prior art;

FIG. 2 shows a graph of a schematic CR function and spectrum for ashutter and a laser diode light source respectively comprised in thecamera shown in FIG. 1, in accordance with prior art;

FIG. 3 shows a graph of time lines illustrating gating of a 3D camera,in accordance with prior art;

FIG. 4 shows a graph that illustrates wavelength shifts of the spectrumof the laser diode light source relative to the CR function of theshutter shown in FIG. 1 that results from local heating of the lightsource, in accordance with prior art;

FIG. 5 shows a graph of a bias error in distance measurements to thescene provided by the camera shown in FIG. 1, in accordance with priorart;

FIG. 6 schematically shows a 3D gated camera comprising a VCSEL lightsource, in accordance with an embodiment of the technology;

FIG. 7 shows a graph that illustrates wavelength shifts of the spectrumof the VCSEL light source of the camera shown in FIG. 6 that resultsfrom local heating of the light source, in accordance with an embodimentof the technology; and

FIG. 8 shows a graph that illustrates improvement of a bias error indistance measurements to a scene provided by the camera shown in FIG. 6,in accordance with an embodiment of the technology.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gated 3D camera 20 being used toacquire a 3D image of a scene 30 having objects schematicallyrepresented by objects 31 and 32.

Camera 20, which is represented very schematically, comprises a lenssystem, represented by a lens 21 and a photosurface 22 having pixels 23on which the lens system images the scene. The camera comprises ashutter 25 for gating photosurface 22 on or off, which is controllableto selectively have low or high transmittance. Shutter 25 is said to be“closed” when it has low transmittance for light in its operating bandand gates photosurface 22 off, and is said to be “open” when it has hightransmittance for light in its operating band and gates the photosurfaceon. A “gate” refers to a period during which photosurface 22 is gated onby shutter 25 and the photosurface receives light transmitted throughthe shutter.

Camera 20 optionally comprises a light source 26, typically an array ofedge emitter laser diodes 27 that are controllable to illuminate scene30 with a train of transmitted pulses of light at wavelength in theshutter operating band. The light pulse train is schematicallyrepresented in FIG. 1 by a train 40 of hat pulses 41 each pulseassociated with an overhead arrow leaving the light source 26.Optionally, the shutter operating band is an IR band of light. Acontroller 24 controls pulsing of light source 26 and operation ofshutter 25 to gate photosurface 22. Function of light pulse train 40 andlight pulses 41 in providing data for distance measurements to scene 30are discussed below.

Typically, during operation of camera 20, light source 26 is controlledto repeatedly illuminate scene 30 with a train of light pulses 40.During each train 40 of light pulses, light source 26 generates anddissipates heat in the camera and the light source temperature cyclesbetween a minimum and maximum temperature. Because of repeated cycles ofheat generation and dissipation, shutter 25 is heated to an elevatedoperating temperature that is greater than the ambient temperature ofthe camera environment and is bracketed by the minimum and maximum lightsource temperatures. For an ambient temperature of about 30° C., theshutter operating temperature may be about 50° C., and the light sourcetemperature may cycle from about 20° C. below to about 20° C. above theshutter operating temperature during generation of a light pulse train40.

Since both an operating band of a shutter and a spectrum of a lightsource change with change in temperature at which they operate, during alight pulse train 40, as temperature of light source 26 varies relativeto the operating temperature of shutter 25, the spectrum of light fromthe light source wavelength-shifts relative to the CR. As a result, theCIN of shutter 25 and light source 26 varies during each light pulsetrain. For reliable operation of camera 20 it is advantageous that CINbe maximized, and that variance of the CIN relative to its maximum thatare caused by differences in operating temperatures of light source 26and shutter 25 be relatively small. To this end, the light source andshutter are matched so that light from the light source is atwavelengths in the operating band of the shutter and generally, at leastin a midpoint temperature of the light source heat cycle, the CIN of thecamera is a maximum. Also, the light source and shutter are generallyformed from a same semiconductor material so that they have similartemperature dependence.

FIG. 2 shows a graph 60 of a schematic contrast ratio, i.e. CR, function70, shown with a dashed line, for shutter 25 and a spectrum 80, shown ina solid line, for light source 26 comprising edge emitting laser diodes27, as a function of wavelength. Both shutter 25 and laser diodes 27 areassumed to be made from GaAs and matched to have a maximum CINoptionally when operating at a same temperature of about 50° C.Wavelength is indicated along the abscissa of graph 60 and values for CRare indicted along a left-hand ordinate 61 of the graph. Spectrum 80 isnormalized to its maximum, and relative values for the spectrum areindicated along a right hand ordinate 62 of the graph. Normalizedcontrast intensity, CIN, for the shutter CR 70 and light source spectrum80 at 50° C. is equal to about 13.4. For GaAs, typically, spectrum 80and CR function 70 shift by about 0.25 nm/° C.

To acquire a 3D image of scene 30 with camera 20 shown in FIG. 1,controller 24 controls light source 26 to illuminate scene 30 with atrain 40 of light pulses 41, which pulses are assumed to have a pulsewidth τ. Light from each light pulse 41 is reflected by features inscene 30 and some of the reflected light is incident on camera 20 andcollected by lens 21. Hat pulses 50 shown in dashed lines and associatedwith overhead arrows pointing toward camera 22 in FIG. 1 schematicallyrepresent reflected light from transmitted pulses 41 that reaches camera20. Following emission of each at least one light pulse 41, controller24 controls shutter 25 to gate on photosurface 22 at a suitable timerelative to a time at which the light pulse is emitted to receive andimage on the photosurface light 50 reflected from the transmitted lightpulse that is collected by lens 21. Amounts of light 50 imaged on pixels23 of the photosurface during gates of camera 20 are used to determinedistances to features of scene 30 that are imaged on the pixels andprovide thereby a 3D image of the scene.

Various methods of gating a 3D camera and acquiring distances tofeatures in scene 30 are described in the patents referenced above andin PCT Patent application PCT/IL2007/001571, the disclosure of which isincorporated herein by reference. FIG. 3 shows a graph 100 of time lines101, 102 and 103, which illustrate illuminating scene 30 with train 40of light pulses 41 and gating of photosurface 22 by shutter 25 toacquire distances to features in the scene, in accordance with arelatively simple gating scheme. Time lines 101, 102 and 103 graphicallyillustrate timing of gates of shutter 25 relative to a radiation timet_(O), of an arbitrary light pulse 41 in train 40 of pulses radiated bylight source 26 to illuminate scene 30. The single light pulse isrepresented by a shaded hat pulse 41 with overhead arrow pointing to theright along time line 101. Time lines 102 and 103 graphically show gatesof shutter 25. The gating profile illustrated in FIG. 3 is typicallyrepeated for each pulse 41 in pulse train 40.

Let a given feature in scene 30 that is imaged on a corresponding pixel23 (FIG. 1) in photosurface 22 be located at a distance D_(f) fromcamera 20. Photons in a light pulse 50 reflected by the given featurefrom radiated light pulse 41 first reach camera 20 at a time,t_(γ1)(D_(f)), that is dependent on the distance D_(f). Reflected lightpulse 50 has a same width, τ, as radiated light pulse 41, and a lastphoton in reflected light pulse 50 reaches the camera from the givenfeature at a time t_(γ2)(D_(f))=t_(γ1)(D_(f))+τ. Reflected light pulse50 is represented along time lines 101, 102 and 103 by a shaded hatpulse 50 shown in dashed lines with an overhead arrow pointing to theleft along the time lines.

A hat pulse 110 along time line 102 schematically represents a gate,hereinafter a “timing gate 110”, during which controller 24 (FIG. 1)controls shutter 25 to gate on photosurface 22 and register light thatreaches camera 20. Timing gate 110 is, optionally, relatively short andhas a gate width equal to τ, starts at a time t_(gs) following t_(O) andends at a time t_(ge)=(t_(gs)+τ). Reflected pulse 50 is shown along timeline 102 for convenience to show clearly a temporal relation betweenreflected pulse 50 and timing gate 110. Reflected pulse 50 overlapstiming gate 110 for a time-portion, T, of the gate. Assume that duringtime T pixel 23 (FIG. 1) that images the given feature registers anamount, “Q”, of charge responsive to light from the given feature thatis incident on the pixel. Assume further that if reflected pulse 50 weretemporally congruent with gate 110 (i.e. if a first photon and a lastphoton in the reflected pulse were to reach camera 20 at times t_(gs)and t_(ge) respectively) the pixel imaging the given feature wouldregister a total amount, “Q_(O)”, of light. Then T=τQ/Q_(O) and if crepresents the speed of light, distance D_(f) of the feature from camera20 may be given by the following expressions:

D _(f)=(c/2)[t _(gs)−(τ)(1−Q/Q _(O))] if t _(gs) ≦t _(γ2)(D _(f))≦t_(ge); and   (1)

D _(f)=(c/2)[t _(gs)+(τ)(1−Q/Q _(O))] if t _(gs) ≦t _(γ1)(D _(f))≦t_(ge).   (2)

From equations (1) and (2) it is noted that for gate 110 having startand end times t_(gs) and t_(ge) respectively, distances are provided forfeatures in scene 30 located in an “imaging slice” of the scene of widthCT centered at a distance (c/2)t_(gs) from camera 20.

For convenience of presentation, let the distance (c/2)t_(gs) of thecenter of an imaging slice is represented by “D_(C)”, and a distance,(c/2)t_(gs)(1−Q/Q_(O)), that is subtracted or added respectively inequations (1) and (2) to D_(C) to provide D_(f) for a feature isrepresented by ΔD_(f). Then equations (1) and (2) may be written:

D _(f) =D _(C) −ΔD _(f) if t _(gs) ≦t _(γ2)(D _(f))≦t _(ge); and   (3)

D _(f) =D _(C) +ΔD _(f) if t_(gs) ≦t _(γ1)(D _(f))≦t _(ge).   (4)

Optionally, Q_(O) is determined by gating camera 20 on for a relativelylong gate, hereinafter a “normalization gate”, which is represented by ahat pulse 112 along time line 103. Gate 112 optionally has a gate widthequal to 3τ and begins at a time (t_(gs)−τ) following transmission timet_(O) of a pulse 41, in the train 40 of light pulses. The width andtiming of the normalization gate are determined so that for everyfeature in scene 30 for which light is registered on a pixel ofphotosurface 22 during timing gate 110, normalization gate 112 willprovide a value for Q_(O). A reflected pulse 50, reflected by the givenfeature at distance D_(f) from camera 20 is shown along time line 103 toshow relative timing of the reflected pulse and normalization gate 112.The reflected pulse falls completely within the temporal boundaries ofnormalization gate 112 and all the light in the reflected pulse isregistered by the pixel and provides a measure of Q_(O). It is notedthat camera 20 may be gated on for timing and normalization gates 110and 112 responsive to times at which different light pulses 41 areradiated and therefore register light from different reflected lightpulses 50.

There are various ways to determine which equations (1) or (2) to applyfor a given reflected pulse 50. For example, timing gate 110 mayoptionally be divided into two contiguous gates, a “front” gate and a“back” gate, each having a gate width equal to τ/2. If a pixel registersa greater amount of light during the front gate or back gate, then forthe feature of scene 30 that is imaged on the pixel equations (1) or (2)respectively apply.

Equations (1) and (2) assume that the CR for light provided by lightsource 26 is infinite and when shutter 25 is closed, no light from lightsource 26 is transmitted through the shutter to reach photosurface 22.However, as discussed above and shown in FIG. 2, CR for shutter 25 isfinite, and by way of example, in FIG. 2, CR has a maximum value ofabout 16. As a result, light in reflected light pulses 50 from a featurein scene 30 that reaches camera 20 when the shutter is closed leaksthrough the shutter and “contaminates” values of Q registered by a pixelin photosurface 22 that images the feature. The contamination in generalgenerates an error in the distance D_(f) determined for the feature byequation (1) or (2) and the error increases as CR decreases.

For example, as noted above, (τ)Q/Q_(O) is duration of overlap time, T,during which photons in a reflected pulse 50 from a given feature inscene 30 reaches camera 20 during timing gate 110 and shutter 25 hasmaximum transparency. A period (τ)(1−Q/Q_(O)) is therefore a duration oftime that photons reach the camera from the reflected pulse when shutter25 is off and the shutter has transparency reduced from the maximum by afactor 1/CR.

Let a total amount of light collected by a pixel 23 that images thegiven feature with light from a pulse 50 during timing gate 110 berepresented by Q*. Q* can reasonably accurately be estimated inaccordance with an expression,

Q*=Q+[Q _(O) /CIN](1−Q/Q _(O)),   (5)

where CIN is the normalized contrast intensity CI for the CR of shutter25 and spectrum of reflected light pulse 50.

Let D_(f)* represent distance determined for the given feature using theamount of light Q* registered by the pixel that images the givenfeature. Then equations (1) and (2) give

D _(f)*=(c/2)[tg _(s)−(τ)(1−[Q+[Q _(O) /CIN](1−Q/Q _(O))]/Q _(O))] if tg_(s) ≦t _(γ2) ≦tg _(e); and   (6)

D _(f)*=(c/2)[tg _(s)+(τ)(1−[Q+[Q _(O) /CIN](1−Q/Q _(O))]/Q_(O))] if tg_(s) ≦t _(γ1) ≦tg _(e).   (7)

Or

D _(f) *=D _(C) −ΔD _(f)+(c/2)(τ)(1−Q/Q _(O))/CIN)] if tg _(s) ≦t _(γ2)≦tg _(e); and   (8)

D _(f) *=D _(C) +ΔD _(f)−(c/2)(τ)(1−Q/Q _(O))/CIN)] if tg _(s) ≦t _(γ1)≦tg _(e).   (9)

Equations (8) and (9) indicate that for features in the imaging sliceassociated with timing gate 110, distance to a given feature determinedfrom charge registered by a pixel 23 that images the feature iserroneously biased towards the center of the imaging slice by a biaserror, “δD” having magnitude,

δD=(c/2)(τ)/CIN)(1−Q/Q _(O)),   (10)

In equation (10), Q is an amount of charge that would be registered bythe pixel imaging the feature were CR of shutter 25 equal to infinity.

Whereas determined distances D_(f)* can generally be corrected for CRand therefore CIN, being finite, it can be difficult to make suchcorrections accurately because, as noted above, when light source 26radiates a train 40 (FIG. 1) of light pulses 41, the light sourceundergoes local cyclical heating with each pulse it radiates. The localheating generates cyclical temperature differences between a temperatureat which light source 26 operates and an operating temperature at whichshutter 25 operates. The temperature difference can be as much as 20° C.As a result, spectrum 80 shown in FIG. 2 can shift by as much as 5 nmduring operation of camera 20 to determine distances to features inscene 30. The shift with temperature difference can result in asubstantial decrease in CIN of shutter 25 and a concomitant increase inbias error δD.

By way of example, FIG. 4 shows a graph 120 of CR function 70 forshutter 25 and spectrum 80 of light source shown in graph 60 (FIG. 2)for camera 20 operating at an operating temperature of 50° C. andspectra 81 and 82 for the light source that are shifted by local heatingresulting from operation of the light source. Spectra 81 and 82 areassumed to prevail at temperatures of 30° C. and 70° C. respectively.From graph 120, it is readily seen by noting amounts by which spectracurves 81 and 82 overlap CR curve 70, that CIN for spectra 81 and 82 aresubstantially reduced relative to a maximum CIN, which occurs forspectra 80. CIN has values equal of about 13.4, 6, and 5, for spectra80, 81, and 82 respectively.

FIG. 5 shows a graph 130 of a bias error δD in distance to a givenfeature of scene 30 that could be generated assuming a CIN equal toabout 6, i.e. a CIN that might occur for a temperature differencebetween light source 26 and shutter 25 equal to about 20° C. Graph 130assumes a pulse width τ for light pulse 41 (FIG. 1 and FIG. 3) equal to10 ns and a gate width for gate 110 (FIG. 3) equal to the pulse width.An imaging slice of scene 30 therefore has a width equal to about 2.7 m.In graph 130, a curve 131 shows δD as a function of displacement ΔD_(f)from the center of the imaging slice. From the graph, it is seen thatthe error bias δD is substantially equal to zero for features at thecenter of the imaging slice, grows linearly with distance from the slicecenter, and is equal to about 17% of ΔD_(f).

In accordance with an embodiment of the technology, a gated 3D cameracomprises a light source having an array of VCSELs that providesimproved matching of the light source to the camera shutter and reduceδD. VCSELs have relatively narrow spectra that are substantially lesssensitive to temperature changes than the spectra of laser diodes. Atypical edge emitting laser diode may have a spectrum that is about 4 nm(FWHM) wide, and as noted above, may exhibit a spectrum shift by about0.25 nm/° C. A typical VCSEL on the other hand typically has a spectrumbetween about 0.5 nm and 1 nm wide that shifts by about 0.07 nm/° C.However, VCSELs, which are typically used for relatively low energycommunication applications, do not in general provide sufficient opticalenergy for use in a light source of a gated 3D camera.

VCSELs in a light source, in accordance with an embodiment of thetechnology, are modified to increase their optical energy output bybroadening their laser cavities. Whereas, broadening the laser cavity ofa VCSEL causes the width of the spectrum of the VCSEL to increase, thespectrum is still generally substantially narrower than that typicallyprovided by a conventional edge emitting laser diode. As a result,modified VCSELs in a light source in accordance with an embodiment ofthe technology, provide both sufficient power for advantageous use in agated 3D camera and improved matching to the camera shutter.

Conventional VCSELs typically have laser cavity cross sections ofdiameter about equal to or less than 15 microns. Optionally, VCSELS in agated 3D camera light source in accordance with an embodiment of thetechnology comprise a relatively large laser cavity having a crosssection characterized by a width, e.g. a diameter, greater than or aboutequal to 20 microns. Optionally, the VCSEL laser cavity width is greaterthan or about equal to 25 microns. Optionally, the width of the spectrumis greater than or about equal to 2.5 nm. In some embodiments of thetechnology the spectrum width is greater than or about equal to 3 nm.

FIG. 6 schematically shows a gated 3D camera 220 that has a light source226 comprising VCSELs 227, in accordance with an embodiment of thetechnology. Gated 3D camera 220 is, by way of example, similar to gated3D camera 20 shown in FIG. 1 except for light source 226, whichcomprises VCSELs 227 rather than laser diodes 27. VCSELs 227 haverelatively large laser cavities characterized by, optionally circular,cross sections of diameter equal to about 20 microns. Optionally, theVCSELs are operable to generate IR light characterized by a spectrumhaving FWHM between about 2.5 nm to about 3 nm centered at a wavelengthof about 855 nm when operated at 50° C.

In light source 226, by way of example, VCSELS 227 are configured in arectangular array of 16 rows and 16 columns with a pitch of about 55microns that is mounted to a suitable heat dissipating package 228. Anyof various packages known in the art may be adapted and used in thepractice of the technology for dissipating heat generated by VCSELs 227during their operation. The inventors have determined that light source226 comprising VCSELs 227 and suitable heat dissipating package 228 canbe operated at a power level about equal to or greater than 12 Watts togenerate a train of light pulses suitable for illuminating a scene fordetermining distances to the scene. In some embodiments of thetechnology, the camera is configured so that the light source may beoperated at a power level about equal to or greater than 15 Watts. Insome embodiments of the technology, the power level is about equal to orgreater than 18 Watts.

For temperature differences between light source 226 and shutter 25produced during operation of the light source to provide a train oflight pulses, the spectrum of light provided by the light sourceexhibits relatively small wavelength shifts and relatively large valuesfor CIN. As a result, camera 220, in accordance with an embodiment ofthe technology can provide distance measurements for a scene withsubstantially smaller bias errors δD than the bias errors shown in FIG.4 for camera 20 (FIG. 1).

FIG. 7 shows a graph 250 of spectra 251, 252, and 253 for VCSELs 227formed from GaAs for temperatures 30° C., 50° C., and 70° C. and CRcurve 70 for shutter 25 at 50° C. shown in graph 120 of FIG. 4. Fromgraph 250 it is seen that for temperature differences of 20° C. that canbe generated between light source 226 and shutter 25 during operation ofthe light source, the spectra shift by relatively small amounts relativeto CR curve 70. Assuming that for each degree centigrade in operatingtemperature of VCSEL 227 (FIG. 6), the VCSEL spectrum shifts by 0.07 nm,spectra 251 and 253 are shifted by about 1.4 nm relative to CR curve 70.The shifts of spectra 251 and 253 relative to CR curve 70 that areexhibited by light source 226 are substantially smaller than thoseexhibited for the same temperature differences by spectra 81 and 82respectively of light source 26 shown in FIG. 4. The CIN values forshifted spectra 251 and 253 for VCSELs 227 and CR curve 70 are alsosubstantially larger than those of corresponding spectra 80 and 82 forlaser diode light source 26.

CIN values for spectra 251, 252 and 253 for light source 226 and CRcurve 70 are estimated to be equal to about 12.5, 13.8, and 12.7respectively. The CIN values for VCSEL light source 226 at about 30° C.or about 70° C. when shutter 25 is at about 50° C. are larger by morethan a factor of about 2 than those for laser diode light source 26 attemperatures of about 30° C. or about 70° C. when shutter 25 is at about50° C. Ratios of CIN values for VCSEL light source spectra 251 and 253relative to laser diode light source spectra 81 and 82 respectively arelarge because the VCSEL spectra are relatively narrow, and for a sametemperature difference relative to temperature of shutter 25, they areshifted by substantially smaller amounts than the spectra of laserdiodes.

FIG. 8 illustrates advantageous improvements in bias errors δD that canbe provided by VCSEL light source 226 and camera 220, in accordance withan embodiment of the technology. FIG. 8 shows a graph 260 that isidentical to graph 130 shown in FIG. 5 with an addition of a δD curve261 that shows δD as a function of ΔD_(f) for 3D camera 220, comprisingVCSEL light source 226, in accordance with an embodiment of thetechnology. Curve 261 assumes a same temperature difference of 20° C.between VCSEL light source 226 and shutter 25 as is assumed for δD curve131 between laser diode light source 26 and shutter 25. Graph 260 showsthat δD for 3D camera 220 is substantially smaller than that for 3Dcamera 20 and is about half of that for 3D camera 20. Whereas bias errorδD is equal to about 17% of ΔD_(f) for laser diode light source 26 andcamera 20 the bias error is reduced by about ½ and is equal to about 8%of ΔD_(f) for VCSEL light source 226 and camera 220, in accordance withan embodiment of the technology.

In the description and claims of the application, each of the words“comprise” “include” and “have”, and forms thereof, are not necessarilylimited to members in a list with which the words may be associated.

The technology has been described using various detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to limit the scope of the technology. The described embodimentsmay comprise different features, not all of which are required in allembodiments of the technology. Some embodiments of the technologyutilize only some of the features or possible combinations of thefeatures. Variations of embodiments of the technology that are describedand embodiments of the technology comprising different combinations offeatures noted in the described embodiments will occur to persons withskill in the art. It is intended that the scope of the technology belimited only by the claims and that the claims be interpreted to includeall such variations and combinations.

1. A camera for determining distances to a scene, the camera comprising: a light source comprising at least one vertical cavity surface emitting light emitting laser (VCSEL) controllable to illuminate the scene with a train of pulses of light having a characteristic spectrum; a photosurface; optics for imaging light reflected from the light pulses by the scene on the photosurface; and a shutter operable to gate the photosurface selectively on and off for light in the spectrum.
 2. A camera according to claim 1 wherein the light source has a characteristic spectrum with a FWHM width equal to or greater than about 1.5 nm.
 3. A camera according to claim 1 wherein the characteristic spectrum has a FWHM width equal to or greater than about 2.0 nm.
 4. A camera according to claim 1 wherein the characteristic spectrum has a FWHM width equal to or greater than about 2.5 nm.
 5. A camera according to claim 1 wherein the VCSEL has a lasing cavity characterized by a diameter about equal to or greater than 20 microns.
 6. A camera according to claim 1 wherein the VCSEL has a lasing cavity characterized by a diameter about equal to or greater than 25 microns.
 7. A camera according to claim 1 wherein a normalized convolution of shutter contrast ratio (CR) and characteristic spectrum is greater than or equal to about 10 for a temperature difference between the shutter and light source less than or equal to about 20° C.
 8. A camera according to claim 1 wherein a normalized convolution of shutter contrast ratio (CR) and characteristic spectrum is greater than or equal to about 12 for a temperature difference between the shutter and light source less than or equal to about 20° C.
 9. A camera according to claim 1 wherein the light source operates at a power level about equal to or greater than 12 Watts to illuminate the scene with the train of light pulses.
 10. A camera according to claim 1 wherein the power level is about equal to or greater than 15 Watts.
 11. A camera according to claim 1 wherein the power level is about equal to or greater than 18 Watts.
 12. An apparatus for determining distances to a scene, the apparatus comprising: a light source having an array of VCSELs provides improved matching of the light source to the camera shutter reduced bias errors, controllable to illuminate the scene with a train of pulses of light having a characteristic spectrum; a photosurface; a lens system for imaging light reflected from the light pulses by the scene on the photosurface; and a shutter operable to gate the photosurface selectively on and off for light in the spectrum.
 13. An apparatus according to claim 12 wherein the light source has a characteristic spectrum with a FWHM width equal to or greater than about 1.5 nm.
 14. An apparatus according to claim 13 wherein the VCSEL has a lasing cavity characterized by a diameter about equal to or greater than 20 microns.
 15. An apparatus according to claim 14 wherein a normalized convolution of shutter contrast ratio (CR) and characteristic spectrum is greater than or equal to about 10 for a temperature difference between the shutter and light source less than or equal to about 20° C.
 16. An apparatus according to claim 15 wherein the light source operates at a power level about equal to or greater than 12 Watts to illuminate the scene with the train of light pulses.
 17. A camera for determining distances to a scene, the camera comprising: a light source having an array of VCSELs providing improved matching of the light source to the camera shutter and being controllable to illuminate the scene with a train of pulses of light having a characteristic spectrum, the light source has a characteristic spectrum with a FWHM width equal to or greater than about 1.5 nm, and a lasing cavity characterized by a diameter about equal to or greater than 20 microns; a controller coupled to the light source; a photosurface comprising a CCD or CMOS photosensitive surface; a lens system for imaging light reflected from the light pulses by the scene on the photosurface; and a shutter operable to gate the photosurface selectively on and off for light in the spectrum. 